As the lumbar spine ages, disc degeneration occurs. This degeneration causes a reduction in the vertical height of the disc, and a diminution of its viscoelastic properties. The profile of the spine also changes with age. The swayback curvature of youth becomes the flat-back of old age. As a result, arthritic changes occur in the facet joints due to the increased biomechanical stress on the posterior side of the spine.
With a recent increased understanding in the biomechanics of the spine, it is acknowledged that maintenance of the normal curvature of the lumbar spine is preferable. For example, it is now known that the instantaneous center of rotation in the lumbar spine is 7 mm or 8 mm anterior to the posterior edge of the vertebral body, and it is approximately 2 cm anterior to the posterior elements and the facet joints. These joints are arranged in a cross-sectional “J” shape, and are designed to stabilize the spine and transmit the biomechanical forces from one vertebra to another.
These joints are an integral part of the stability of the motion segment. The facet joints transmit torsional force, facilitating normal gait. So, when spinal fusions are considered, it is important to re-establish the normal biomechanical arrangement, and to restore the sagittal profile of the spine to obtain optimal results. Arthritic changes in the facet joints following disc degeneration can cause mechanical back paint. If they become excessive, these arthritic changes can cause spinal stenosis.
Historically, some of the first attempts to attain spinal fusion, or spinal arthrodesis, utilized a method whereby the facet joints were stabilized by fusion. Fusion of the motion segment entails removal of the cartilage from the facet joint, and then packing bone into this joint to obtain immobility. In so doing, forces were transmitted from one segment to the next by a bony connection rather than by a flexible connection.
Another prior art method of stabilizing the facet joint is shown in FIG. 1. In this method, a screw was inserted from a dorsal and medial approach, through the facet joint, running from the medial to lateral side of the spine. The screw passed through the inferior facet of the vertebra above, and as it crossed the joint, it penetrated the superior facet of the vertebra below. However, this original technique is flawed in that the surgeon is unable to visualize where the tip of the screw comes to rest. The greatest complication of this technique is that the tip of the screw can end up in the vicinity of the exiting segmental nerve root. Nerve root encroachment can, in turn, produce serious radicular pain, which is the major complication of this original method. Accordingly, a need exists for a method of stabilizing facet joints which improves the safety of the stabilizing screw.